A deployable diverging bell is used in particular in a rocket engine of an upper stage (e.g. second or third stage). In such an application, it is necessary to deliver a very large amount of thrust, where the amount of thrust depends essentially on the rate at which gas is ejected and on the speed of ejection. In order to optimize those parameters, it is necessary to have a diverging bell that presents a very large expansion ratio, and thus a large outlet diameter. It is therefore necessary to have a diverging bell that is long, which is not compatible with the small amount of space available, in particular when the diverging bell is fitted to a thruster of the second or third stage of a rocket.
For applications of this type, a deployable diverging bell is therefore used, i.e. a bell that is capable of adopting a retracted position, thereby giving it a short length, and capable of being lengthened by deploying one or more movable portions, in order to adopt its utilization configuration.
FIG. 1 shows a prior art deployable diverging bell that comprises a first portion 10 of the diverging bell that is stationary and that is connected to a stationary support 12 of the thruster, and a second portion 14 of the diverging bell that is movable in a longitudinal travel direction X-X′ that corresponds to the direction of the longitudinal axis of the diverging bell. FIG. 1 shows the diverging bell in its deployed, utilization configuration, the upstream edge 14A of the portion 14 being connected to the downstream edge 10A of the portion 10.
As stated above, the stationary portion 10 of the diverging bell is stationary relative to the support 12. The deployment mechanism for deploying the movable portion 14 in the example shown comprises a wormscrew 20 that has its end remote from the movable portion 14 of the diverging bell supported by arms 18 and 16 via a connection part 22, these arms projecting outwardly and being themselves fastened to the stationary support 12 at their ends remote from the wormscrew 20. The deployment mechanism also includes a nut 24 in which the wormscrew is engaged, the nut 24 being supported by a fastener fitting 26 fastened to the upstream edge 14A of the movable portion 14 of the diverging bell. In the example shown, three deployment mechanisms of this type are provided.
It can be understood that the fastener fitting 26 has a first fastener zone 26A to which the nut 24 forming part of the deployment mechanism is fastened, and a second fastener zone 26B that is fastened to the movable portion 14 of the diverging bell. Overall, the fitting 26 is stirrup-shaped, and its second fastener zone 26B is in the form of a transverse strip extending in the circumferential direction of the portion 14 of the diverging bell. In known manner, this fastening may be performed by adhesive and by screws. The “transverse strip” shape of the second fastener zone 26B stiffens the fastener zone between the fastener fitting 26 and the movable portion 14 of the diverging bell. This leads to difficulty insofar as the diverging bell, as a whole, is subjected to very high temperatures and to very high levels of vibration. In the fastener zone of each fitting 26, the second portion 14 of the diverging bell is thus stiffened on a portion of the periphery of its upstream edge corresponding to the length of the “strip” 26B, such that its response to the various stresses to which it is subjected is non-uniform around said upstream edge. In particular, it may tend to deform by ovalizing under the combined effect of high temperatures and vibration, while the fastener “strip” 26B does not follow the ovalization. This results in a risk of the fastener breaking, since the fastening means such as adhesive or screws are subjected to particularly high traction forces, and this also leads to risks of the movable portion 14 of the diverging bell being damaged.